In certain video disc systems video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup, supported by a pickup arm, engages the spiral groove and includes a conductive electrode which establishes a capacitance with the conductive coating and the dielectric deposit of the disc record. When the disc record is rotated, the electrode/disc capacitance varies in response to the geometric variations in the bottom of the spiral groove passing beneath. The capacitance variations are converted to electrical variations by a suitable signal processing circuitry coupled to the signal pickup electrode. The output signal of the signal processing circuitry may be coupled to a conventional television receiver for reproduction. The other end of the pickup arm is releasably secured by a coupler to a support member of a supporting structure of the playback system. A system of the aforementioned type is described in detail in the U.S. Pat. No. 3,842,194, issued on Oct. 15, 1974, to J. K. Clemens.
Video disc systems of the aforementioned type generally utilize disc records having groove densities in the order of four to eight thousand groove convolutions per inch. A typical video disc record of this type may have a groove convolution spacing in the order of 3.5 microns. The fragile walls of relatively narrow grooves of the disc record cannot be dependably relied upon to pull the weight of the pickup arm apparatus, around the pickup arm pivot support, across the entire recorded surface of the disc record. Also, in video disc systems utilizing the variable capacitor concept, it is desirable for accurate reproduction of the prerecorded signals that the signal pickup electrode maintain a substantially constant attitude in the spiral groove. Stated differently, the position and the angular orientation of the signal pickup electrode in relation to the information track in the spiral groove must be held relatively constant. Therefore, the supporting structure includes a radial feed drive mechanism for traversing the supported end of the pickup arm in proper time relationship with the radial motion of the signal pickup tip engaged in the spiral groove so as to continuously maintain the longitudinal axis of the pickup arm substantially tangential to the spiral groove at the point of engagement. Reference may be made to the U.S. Pat. No. 3,870,835 issued to F. R. Stave on Mar. 11, 1975 and assigned to the present assignee, for an illustration of a suitable radial feed drive mechanism for providing the indicated radial motion.
Further, in the above-mentioned type video disc systems it has been recognized that the relative motion between the disc record and the signal pickup must be maintained at a predetermined speed, and within specified tolerance limits (e.g., 450 rpm, .+-.0.01 percent), in order to obtain high fidelity of reproduction of the prerecorded signals. The predetermined speed and the specified tolerance limits are also necessary to assure that the horizontal and vertical synchronizing information is stable and within the lockup range of the deflection circuits of the television receiver. Moreover, when the prerecorded information is a color television signal with chrominance information recorded as a modulated carrier signal, the recovered signal must be stable and within the lockup range of the color processing circuits of the playback system in order to reduce color phase distortion.
The disc record/signal pickup relative speed may be maintained at the predetermined speed, and within the specified tolerance limits by rendering the pickup arm support subject to cyclical, translatory motion along the longitudinal axis of the pickup arm in a manner that opposes deviations of the instantaneous relative speed from the predetermined speed. Illustratively, the means for imparting translatory motion to the support may be of the type disclosed in the U.S. Pat. No. 3,711,641, issued to R. C. Palmer on Jan. 16, 1973, entitled "VELOCITY ADJUSTING SYSTEM", and assigned to the present assignee.
Additionally, in an advantageous pickup arm arrangement for video disc systems of the aforementioned type, the pickup arm is made of conductive material and is enclosed in a conductive cage. The conductive pickup arm and the surrounding conductive cage serve respectively as an inner and outer conductors of an electrical transmission line. The transmission line is capacity end loaded at the pickup arm supported end by a series combination of capacitances which include an air dielectric capacitor and a voltage variable capacitor (for tuning purposes). The transmission line is also capacitor end loaded at the signal pickup end by a combination of capacitances which include the signal pickup electrode/disc record conductive coating variable capacitance. The transmission line and associated capacitances form a tuned circuit with a resonant frequency subject to variations as the signal pickup electrode/disc conductive coating capacitance varies. The tuned circuit is excited with UHF oscillations from a fixed frequency oscillator of the signal processing circuitry operating at a frequency (e.g., 915 MHz) within an ISM-allocated band. As the resonant frequency of the tuned circuit varies, the resultant UHF oscillations amplitude variations are detected by a detector of the signal processing circuitry to recover the prerecorded information. Reference may be made to the U.S. Pat. No. 3,872,240 issued to D. J. Carlson, et al., on Mar. 18, 1975 and assigned to the present assignee, for a more detailed description of the aforesaid transmission line and the associated circuitry.
Thus, in systems of the aforementioned type the pickup arm performs a dual function: (1) it mechanically supports the pickup electrode in signal transfer relation with the disc record during playback, and (2) it forms an electrical transmission line with the surrounding conductive cage so that the prerecorded signals may be recovered. It is advantageous to have a pickup arm which effectively serves the mechanical and electrical functions. First, in connection with the pickup arm's mechanical function, it is desirable to reduce the angle (hereinafter, angle y) included between (1) a line joining the signal pickup tip and the effective pivot (for the vertical arcuate motion of the signal pickup in the spiral groove), and (2) the disc record surface, in order to reduce random forward and backward displacement of the signal pickup along the disc record spiral groove as a result of random up and down motion of the signal pickup in the spiral groove (e.g., due to warpage, orange peel, etc.). The random shifting of the signal pickup along the spiral groove results in deleterious fluctuation of relative speed therebetween, thereby causing flickering or jitter of the picture projected on the television screen (which is analogous to "wow" in audio playback systems). In other words, the effective pivot must be as near to the disc record as practicable.
Second, in reference to pickup arm's mechanical function, it is preferable to reduce the distance (hereinafter, distance R) between the effective pivot for the signal pickup and the signal pickup tip for two reasons. One, the smaller the distance R, the smaller is the pickup arm apparatus mass which must follow vertical and lateral motion of the signal pickup in the spiral groove (or in other words, the better is the compliance of the pickup arm assembly). Two, the random, forward and backward displacement of the signal pickup along the spiral groove (for a given angle y) is reduced by reducing the distance R.
Third, in connection with the pickup arm's mechanical function, it is advantageous to dispose the center of gravity of the pickup arm substantially in line with the cyclical, translatory motion of the pickup arm support in order to reduce twisting moments on the pickup arm support when the translatory motion imparting means is activated.
Fourth, in reference to the conductive pickup arm's electrical function, it is desirable that electrical parameters (e.g., reactance) of the transmission line remain substantially free from random variations during playback throughout the range of the translatory motion of the pickup arm for accurate reproduction of the prerecorded signals.
Fifth, in connection with the conductive pickup arm's electrical function, it is advantageous to reduce the undesirable stray capacitance between the disc record conductive coating and the conductive pickup arm portion, adjacent to the signal pickup, in order to prevent interference with the conductive pickup arm's transmission line function.
In a copending application of J. S. Fuhrer, filed concurrently, U.S. Pat. application Ser. No. 522,813, entitled "PICKUP ARM ASSEMBLY", and assigned to the present assignee, a pickup arm arrangement is described. In the Fuhrer arrangement the conductive pickup arm has an intermediate portion interconnecting a first end portion and a second end portion. An end of the first end portion is secured to the player supporting structure so as to dispose (1) both the first and the second end portions parallel to the turntable surface of the player, and (2) the second end portion near the turntable surface relative to the first end portion. A signal pickup holder carrying a signal pickup is pivoted to an end of the second end portion remote from the pickup arm supported end in a manner that normally aligns the axes of the holder and the second end portion.
The present invention is concerned with a modification of the pickup arm assembly of the general form described in the aforesaid Fuhrer application to provide pickup arm apparatus of a modified form which may reduce the undesirable stray capacitance between the disc record conductive coating and the conductive pickup arm portion adjacent to the signal pickup and yet satisfactorily meet other mechanical and electrical requirements set out above.